The invention relates to a transmission drive unit. The applicant's subsequent publication DE 10 2009 000 760 A1 discloses a transmission drive unit which has an output element which is produced from metal, in particular from sintered metal, and is at least partially insert molded by an adaptor element designed as a worm wheel. The adaptor element is accommodated within a transmission housing. The adaptor element is mounted rotatably on an axle stub of the housing base, wherein both the axle stub and the adaptor element are composed of plastic. The previously known transmission drive unit which is designed in particular as a sliding roof drive can be produced relatively inexpensively by the adaptor element being injection molded on. The wear resistance of the known transmission drive unit is worthy of improvement in particular under high loads. Furthermore, force is introduced by the adaptor element to the output element over a relatively small axial length, and therefore the driving torque which can be transmitted is restricted.